This invention relates to a system which transmits and receives electromechanically generated high frequency pulses, such as sonar and ultrasound systems, for example. More particularly, it relates to an improvement in which a system uses the amplitude of a return signal as a trigger. And still more particularly, it relates to a improved system with which the arrival time of the signal can be reliably established despite varying amounts of attenuation or distortion of the signals as they pass through the transmission medium.
Commonly, when the amplitude of a transmitted or a received signal exceeds a threshold level, this information is used as a trigger point. Due to the inertial mass of electromechanical transducers, the transmitted and received signal shows an envelope which attains its maximum value after an initial delay maintains this steady state condition for a certain time period, and then decays. In systems in which triggering occurs on the rising edge of the signal envelope, attenuation of the received signal as it passes through the transmission medium can cause a error in determining the elapsed time between transmission and receipt of the signal.